1. Field of the Invention
This invention relates to steering systems for articulated vehicles and particularly to a steering system for effectively combining vehicle articulation steering with pivotable wheel steering such as may be accomplished by wagon-wheel movement of one axle of the vehicle.
2. Description of the Prior Art
It has become common to articulate certain off-road vehicle such as tractor, graders, scrapers, etc. An articulated vehicle has a smaller turning radius than might otherwise be possible and is more versatile in rough terrain, especially if the vehicle includes a four-wheel drive feature. In such vehicles, articulation is typically achieved by means of a pair of hydraulic cylinders mounted between two articulating body portions of the vehicle. For turning in one direction, one cylinder is extended while the other is retracted, and for turning in the other direction, the opposite reactions occur. In such systems, steering is controlled by means of a mechanical steering control unit which is tied to the vehicle wheel and includes a valve for controlling the flow of pressurized hydraulic fluid to and from the articulation cylinders.
In order to obtain even tighter turning radii and other steering benefits for articulated vehicles, articulation has been combined with other conventional pivotable wheel steering schemes. For example, U.S. Pat. No. 3,515,235, granted June 2, 1970 to Kamner, discloses a combination articulated and Ackerman steering system for vehicles. Similarly, U.S. Pat. No. 3,991,847, granted Nov. 16, 1976 to Unruh, shows an articulated vehicle having one axle mounted in a wagon-steer mode in addition to the conventional articulated-steer mode. The control and feedback for prior art systems of this type has been dependent upon mechanical or hydraulic linkages, which are susceptible to gradual fading and locational errors due to fluid leakage and mechanical restraints. One prior art system which at least partially overcomes these problems is found in U.S. Pat. No. 4,565,257, granted Jan. 21, 1986 to Hanson, which discloses a combination articulated and front-wheel Ackerman steering systems. In the combined steering mode of this steering system, a conventional hydraulic steering unit controls the front wheels, and two potentiometers are used to monitor the degree of front-wheel pivot and the degree of articulation, so that an electrical steering control network can operate the independent hydraulic actuator used for articulated steering in a slave-follower relationship with respect to the orientation of the front wheels. This combined steering mode produces enhanced maneuverability for the vehicle since the steering effect of the front wheels is effectively amplified by the substantially simultaneous, proportional movement of the articulated steering system.
While attempts have been made to combine various steering schemes on an articulated vehicle, an effective control system for such combined steering arrangements which utilizes a conventional mechanically-operated steering control unit with variable hydraulic output for smoothly operating both the articulated steering hydraulic actuator means and the pivotable wheel steering hydraulic actuator means has not been achieved in the prior art. Also, no steering system for an articulated vehicle is known which has an effective scheme for efficiently monitoring the various positions and rates of change of the steering elements with respect to one another and controlling further the timing of changes of steering modes of the vehicle in response thereto automatically, so as to provide a vehicle with smooth automatic transitions between the pivotable steering wheel mode and the articulated steering mode. Accordingly, an important object of the present invention is to provide a steering system for an articulated vehicle which overcomes the aforementioned problems and provides the aforementioned capabilities not heretofore achieved in earlier steering systems.